Date Published: April 3, 2014
Publisher: Public Library of Science
Author(s): Pierre Cavailles, Pierre Flori, Olivier Papapietro, Cordelia Bisanz, Dominique Lagrange, Ludovic Pilloux, Céline Massera, Sara Cristinelli, Delphine Jublot, Olivier Bastien, Corinne Loeuillet, Delphine Aldebert, Bastien Touquet, Gilbert J. Fournié, Marie France Cesbron-Delauw, Christopher M. Sassetti.
Natural immunity or resistance to pathogens most often relies on the genetic make-up of the host. In a LEW rat model of refractoriness to toxoplasmosis, we previously identified on chromosome 10 the Toxo1 locus that directs toxoplasmosis outcome and controls parasite spreading by a macrophage-dependent mechanism. Now, we narrowed down Toxo1 to a 891 kb interval containing 29 genes syntenic to human 17p13 region. Strikingly, Toxo1 is included in a haplotype block strictly conserved among all refractory rat strains. The sequencing of Toxo1 in nine rat strains (5 refractory and 4 susceptible) revealed resistant-restricted conserved polymorphisms displaying a distribution gradient that peaks at the bottom border of Toxo1, and highlighting the NOD-like receptor, Nlrp1a, as a major candidate. The Nlrp1 inflammasome is known to trigger, upon pathogen intracellular sensing, pyroptosis programmed-cell death involving caspase-1 activation and cleavage of IL-1β. Functional studies demonstrated that the Toxo1-dependent refractoriness in vivo correlated with both the ability of macrophages to restrict T. gondii growth and a T. gondii-induced death of intracellular parasites and its host macrophages. The parasite-induced cell death of infected macrophages bearing the LEW-Toxo1 alleles was found to exhibit pyroptosis-like features with ROS production, the activation of caspase-1 and IL1-β secretion. The pharmacological inactivation of caspase-1 using YVAD and Z-VAD inhibitors prevented the death of both intravacuolar parasites and host non-permissive macrophages but failed to restore parasite proliferation. These findings demonstrated that the Toxo1-dependent response of rat macrophages to T. gondii infection may trigger two pathways leading to the control of parasite proliferation and the death of parasites and host macrophages. The NOD-like receptor NLRP1a/Caspase-1 pathway is the best candidate to mediate the parasite-induced cell death. These data represent new insights towards the identification of a major pathway of innate resistance to toxoplasmosis and the prediction of individual resistance.
Toxoplasma gondii is a widespread obligate intracellular protozoan parasite. One preeminent aspect of its life cycle is the establishment of a chronic infection in humans and many other vertebrate hosts . Toxoplasmosis is most often asymptomatic depending on the parasite’s ability to elicit host protective immunity . A serious threat to human health can occur under congenital infection or reactivation of a latent infection in immunodeficient patients . Epidemiological studies have indicated that the phenotypic expression of toxoplasmosis depends on the genetic make-up of both the host and the parasite , . Variations in the outcome of Toxoplasma infection after exposure to similar risk factors ,  and twin studies  support a significant role of the human host genetic background in the susceptibility to toxoplasmosis. Nevertheless, genetic studies in human are hampered by both population heterogeneity and environment variability. In experimental conditions, genetic and environmental factors are under control. Rats, like humans, usually develop subclinical toxoplasmosis. This contrasts with the severity of the disease developed in most strains of mice. Interestingly, an unexpected refractoriness to T. gondii infection was found in the LEW rat strain . Compared to susceptible BN rats, infected LEW indeed displayed negative serology and lack of cyst burden in their brain . Refractoriness of LEW rats was found to be a dominant trait dependent on hematopoietic cells . It is associated with the ability of macrophages to restrict parasite proliferation in vitro.
Forward genetics has proved to be a powerful tool to characterize novel biological pathways implicated in host resistance to infection , . In rats, the Toxo1 locus located on chromosome 10  controls the outcome of toxoplasmosis by a still poorly defined mechanism. In the present work, genetic dissection of Toxo1 with a panel of new congenic sub-lines together with haplotype mapping led us to identify a 891 kb interval of rat chromosome 10 that is highly conserved amongst resistant rat strains and that controls both T. gondii infection outcome in vivo and macrophage responses to infection in vitro. We further demonstrated that the Toxo1-mediated refractoriness of macrophages to T. gondii infection is associated with a caspase-1-dependent rapid T. gondii-induced death of both intracellular parasites and host macrophages.